The present invention relates to the field of communication circuits, and in particular, to a bandwidth switching circuit in a power amplifier control circuit.
Communication circuitry and standards are often changing to compliment innovations in the communications industry. One set of standards is based upon the Global System for Mobile communications (GSM). GSM is a digital cellular communications system that has been generally accepted worldwide. The aim of GSM specifications is to describe the functionality and the interface for each component of the system, and to provide guidance on the design of the system. GSM is a Time Division Multiple Access system (TDMA). TDMA involves dividing a single frequency communication channel into a number of timeslots. Each user or communication device utilizes only one out of every few timeslots. First, a small amount of the communication signal from one user is transmitted, and then a second user, and so on until all users on one channel have transmitted. Once all users have transmitted, the cycle repeats.
An example requirement under GSM specifications is that the spectral band utilized for transmission of the communication signal is kept relatively xe2x80x9cpurexe2x80x9d. In other words, the output power of the communication signal cannot be turned on full, instantly, as this results in a broad spectrum signal when examined in the frequency domain. A broad spectrum signal can pollute the neighboring sidebands of other communication signals utilizing the same cell site. Therefore, the output power of the communication signal is accurately ramped up for transmission, and ramped back down under a controlled process. The purpose of the ramp up and down is to transmit the communication signal at a sufficient level without overdriving the signal. Overdriving the communication signal can cause interference with other signals utilizing the same cell site. The communication signal is often ramped up to an output power that is sufficient to transmit the signal given the distance of the user from the cell site. Transmitting the communication signal at a minimum level also limits power consumption.
Another innovation for GSM networks includes the EDGE system (Enhanced Data Rate for Global System for Mobile Communication Evolution). For EDGE, the data throughput in existing GSM networks has been increased by introducing a second modulation format into the GSM system. EDGE uses a format of 8-state phase-shift-keying (8PSK) modulation (3b/symbol) as opposed to the Gaussian minimum-shift-keying (GMSK) modulation format (1b/symbol). Where GMSK is a constant-envelope modulation format, 8PSK uses phase and amplitude modulation. This new format impacts the design of modulators, amplifiers, and regulators.
The present invention is directed to communication signal amplifier circuits. More particularly, the present invention is directed to a power amplifier control circuit that includes a bandwidth switching circuit. The power amplifier control circuit includes a control loop for providing a control signal to a power amplifier. A communication signal is amplified by the power amplifier in response to the control signal.
According to one aspect, the bandwidth switching circuit switches the loop bandwidth of the control loop according to a bandwidth switching signal. The bandwidth switching circuit includes two configurations: a high loop bandwidth configuration and a low bandwidth configuration. The high bandwidth configuration is selected when the output power is ramping-up and ramping down. The high loop bandwidth allows the control loop to regulate the output power of the communication signal. The low loop bandwidth configuration is selected when the communication signal is transmitted. The low loop bandwidth allows the amplitude modulation portion of the communication signal to be amplified and transmitted without suppression by the control loop. The bandwidth switching circuit is arranged such that the introduction of transient glitches are minimized when switching between the high bandwidth configuration and the low bandwidth configuration.
Briefly stated, an apparatus and method is directed to a power amplifier control circuit that includes a bandwidth switching circuit. The power amplifier control circuit can be used in a GSM EDGE system for controlling the output power of a communication signal. The loop bandwidth of the power amplifier control circuit is high during ramp-up and ramp-down of the output power, and low during transmission of the communication signal. The power amplifier control circuit regulates the output power during power ramping, while preserving the amplitude modulation portion of the communication signal during transmission. The bandwidth switching circuit includes a pair of capacitance circuits that are arranged such that both capacitance circuits charge without affecting the loop bandwidth. The bandwidth switching circuit operates at high speeds associated with a GSM EDGE system, while avoiding transient glitches that may occur due to charging and discharging in the capacitance circuits.
A more complete appreciation of the present invention and its improvements can be obtained by reference to the accompanying drawings, which are briefly summarized below, to the following detail description of presently preferred embodiments of the invention, and to the appended claims.